The presentation needs to "transplant" your idea in the head of your investors or customers. They need to understand what the idea, and then be excited enough about it to buy or invest.

Paying a professional presentation designer to do your slide deck is not a guarantee to succeed. I see many presentations that might not be top notch, but which look professional enough and are clear enough to convey the idea. In those cases I am honest and say that the startup's funds might be better spent elsewhere.

Remember: investors might not like you, investors might not like the idea, investors might not like the market segment, investments might not believe that it will work, there are thousands of potential reasons for "no", and misunderstanding the story and/or the visual quality of the slides is just one of them.